RS virus despite its poor growth in vitro, has been grown to high titer by us and this has allowed us to purify virus by successive cycles of sucrose gradient centrifugation. In addition to genomic 50S RNA, purified virus preparations contain cellular RNA contaminants. The 50S genomic RNA was purified away from these contaminants and 3 feet end labeled by standard procedures. The 50S RNA 32p labeled at the 3 feet end only hybridized to RS virus cDNA clones and not to cellular cDNA clones. By independent hybridization studies the identity of 50S RNA as genomic RS virus RNA was established. Similarly, RS viral nucleocapsids were processed and found to contain substantial amounts of both negative and positive genomes. Studies with parainfluenza 3 virus along such lines yielded similar results.